This invention relates to an exposure apparatus for directly drawing a pattern on a photoresist layer on a substrate. The exposure apparatus uses a plurality of semiconductor lasers as light sources and is applicable to large-sized substrates.
In the conventional lithographic processes for the fabrication of semiconductor devices, it is usual to transfer a mask pattern to a substrate by a projection method for the sake of increased yield of acceptable products and enhanced throughput of exposure apparatus. In the projection method, as the consequence of a trade-off between the resolution of the transferred pattern and the size of an area exposed in each exposure operation, it is prevailing to employ a step-and-repeat technique with an apparatus called a stepper which accomplishes exposure of the entire area of the substrate by repeating the sequence of making an exposure on a limited area of the substrate, then moving the substrate a predetermined distance and making another exposure.
When the step-and-repeat technique is applied to large-sized substrates there arises a problem that the total exposure time greatly increases since the number of repetition of exposure increases in proportion to the square of the diagonal length of the substrate. For example, in the case of forming a regular pattern on a substrate for a liquid crystal display panel of approximately A4 size by using a currently prevailing stepper the total exposure time is about 5 min. If the same step-and-repeat exposure method is employed for the fabrication of a 40-inch display panel which will find a large market in near future, the total exposure time exceeds 50 min which is quite beyond tolerance in commercial fabrication. Besides, with an increase in the substrate size greater difficulties arise in quickly and accurately moving the table or stage carrying the substrate to perform the step-and-repeat process because of an increase in the inertial force acting on the stage.
It is possible to make an exposure on a widened area by using a large aperture lens for mask pattern projection. However, the resolution of the obtained pattern becomes worse, and the exposure apparatus becomes very costly because of great difficulties in the manufacture of the lens.
Another method, which is under development, for pattern fabrication is a direct writing method using a focused energy beam such as electron beam or laser beam to draw a pattern of lines with strokes of the beam spot on a substrate having a photoresist layer. By this method the drawing of a pattern on the substrate is made directly from pattern data in the form of CAD data without using any photographic mask. Therefore, there is a good prospect of practical use of this method, for example, in the fabrication of wiring patterns of application-specified LSIs for which a reduction in development hours is a matter of importance. As to the use of a laser beam in the direct writing method, for example, JP 62-26819 A shows an apparatus for drawing a pattern by splitting an argon laser beam into several beams and controlling the projection of each of the splitted beams with an acoustic-optic modulator. Besides, there is a proposal of using a semiconductor laser as the light source in a direct writing apparatus.
However, there are several problems in the application of the direct writing method to large-sized substrates. First, the total exposure time increases in proportion to the surface area of the substrate because it is usual to make exposure with a projection system comprised of a light source and an objective lens. Besides, this method is inferior to the conventional step-and-repeat method in exposure area per unit time. Therefore, in the case of a large-sized substrate the total exposure time becomes very long so that the throughput of the exposure apparatus becomes insufficient. Furthermore, known exposure apparatus for the direct writing method are complicated and costly mainly because of using deflection scanning optics for the purpose of high speed patterning.